This invention relates to fasteners for joining a pair of panels or structural elements. This is the type of fastener which consists of two parts with one part being secured to one panel and a second part being secured to the other panel. When the fasteners have been secured, the panels are pushed together and the parts interlock forming a concealed, self-locking joint which results from interengagement of the fasteners as the panels are pushed together. Fasteners of this general type have been known and used for many years. They have not been satisfactory because, unless they are of the complex and expensive type or are too large to be effectively concealed, they have the characteristic of becoming loosened by rocking or twisting of the panels. Once this type of deterioration of the joint begins, total failure of the joint can be expected to follow quite rapidly. This type of failure occurs because the fasteners, while effecting a positive lock against the fasteners being pulled apart, do not provide sufficient positive resistance to side or lateral movement because they can rock sideways. Even though this rocking motion is very slight, it quickly results in loosening of the fastener, enlargement of the openings in which the fastener is seated and distortion of the fastener which ultimately results in failure.